One of the chemical pulping processes that is commonly used for making paper is known as the kraft process, which involves cooking of pulping wood chips in an aqueous solution of sodium hydroxide, sodium carbonate and sodium sulfide. Usually, this process is carried out in a digester at temperatures of about 160.degree. C. to 180.degree. C. Following the cooling stage, the liquor is separated from the pulp and subjected to a recovery treatment to recover the chemical and energy values. In many cases, the pulp is further treated with various chemicals to remove the lignins or color bodies from the pulp. This is referred to as the bleaching step, and is crucial to the production of high quality, low color paper materials.
Further, in the various stages of the papermaking process, i.e., pulping, bleaching, etc., large amounts of water are used. While significant improvements have been made in conserving and reusing water in the paper making process, it is still necessary to discharge a certain amount of waste water from the system.
The effluent from a pulp mill includes the dark colored liquor containing lignin degradation products. Since pulp mill plants produce large quantities of this densely-colored effluent, the discharge of this effluent into adjacent streams and bodies of water can cause an objectionable discoloration of the water.
Various processes have been proposed for decolorization of the effluent. Conventional non-biological effluent treatment processes such as ultra-filtration, reverse osmosis, precipitation with lime or alum, and carbon absorption are effective in removing color from such effluent, but are prohibitively expensive for use in treating the quantities of such effluents generated by commercial-size pulp and paper plants. The cost of large amounts of chemical reagents, high maintenance costs and expensive facilities have rendered these proposed solutions impractical.
One of the constituents of the effluent stream is lignin which, if recovered, can be used as a source of energy for generating heat for use in the pulp process. Although several processes have been proposed for recovering heat from components of the effluent stream, these processes have been expensive, since they typically require large quantities of reagents and a substantial capital investment in the heat recovery systems.
For example, it has been the practice to recover heat from the kraft liquor by using it as a fuel in a heat recovery boiler. This process causes the formation of a pool of molten inorganic salts, which are further treated to regenerate the caustic material used in the pulping process.